1. Field of the Invention
The present invention relates to the field of petroleum exploration. Petroleum exploration consists in seeking hydrocarbon reservoirs in a sedimentary basin.
2. Description of the Prior Art
The general procedure comprises:
assessments and geological studies for evaluating the petroleum potential of the sedimentary basin, from available data (outcrops, seismic surveys, drilling data). The goal of this assessment is to:                better understand the architecture and the geological history of the subsoil, notably to study whether hydrocarbon maturation and migration processes may have taken place;        identify the subsoil zones where these hydrocarbons may have accumulated;        establish which zones have the best economic potential, evaluated from the volume and the nature of the hydrocarbons probably trapped (viscosity, rate of mixing with water, chemical composition, . . . ), as well as their operating cost (controlled for example by the fluid pressure and depth); and        
exploratory drilling operations in the various zones having the best potential, in order to confirm or invalidate the previously estimated potential and to acquire new data intended to fuel new and more precise studies.
In some sedimentary basins having a complicated geological history involving many physical processes, or when the volume of data is very large, a simple human assessment intervention is not sufficient to predict the location and the economic interest of reservoirs. A procedure involving software tools of synthesis of the available data and of simulation of the geological history and of the many physical processes that govern it is then applied. This procedure is referred to as “basin modeling”. It provides predictive mapping of the subsoil showing the probable location of the reservoirs, as well as the proportion, the nature and the pressure of the hydrocarbons trapped therein.
Understanding the principles of the genesis of hydrocarbons and their links with the geological history of the subsoil has allowed development of methods for predicting the petroleum potential and the location of reservoirs in sedimentary basins. These methods were based on geological observations made in the field, and expert geologists integrating their knowledge into a reasoning process intended to provide hypotheses for hydrocarbon formation, migration and trapping scenarios for each basin being studied. Exploratory drilling operations were then carried out according to their recommendations, with a success rate of the order of one reservoir discovery out of 10 wells drilled. The information provided by this assessment stage was essentially qualitative.
In the 80s, the development of computer science provided new tools for assisting geologists in this procedure, helping them to provide quantitative and more reliable data, and allowing them to thus increase this success rate. Many computer codes were developed so as to better apprehend and quantify each one of the geological phenomena governing the formation, migration and trapping of hydrocarbons. The following tools can be mentioned:
1. “structural restoration” softwares
2. “basin modeling” softwares.
Structural Restoration
The goal of structural restoration softwares is to construct, visualize and validate a hypothesis relative to the current architecture of the subsoil and to its evolution over the past geological time. The subsoil architecture is described by the shape and the position of the faults and of the interfaces separating the main sedimentary layers (FIG. 1). A structural restoration study is for example started when only imprecise fragmentary data is available on this architecture: geological field observations, seismic surveys or data acquired along wellbores. Schematically, restoration softwares have the functionalities of an interactive online puzzle, in 2 or 3 dimensions. The subsoil is currently assumed to be made up of contiguous, surface or volume elements; the contours of these elements correspond to the sedimentary interfaces and to the faults. Structural restoration reconstructs the successive states of the subsoil architecture during its geological history, starting from its current architecture and going back in time. The architecture of the past states is reconstructed by deforming these elements from their current state, then by displacing them and by arranging them in relation to one another according to the deformation scenario envisaged by the geologist. This tool allows:
converting a conceptual scenario obtained from geological interpretation to a numerically quantified scenario, reproduced by computer;
validating this scenario: the basic validation criterion essentially checks that the scenario envisaged does not violate the condition of perfect contact between the elements; in fact, the presence of vacuum or overlapping between subsoil elements is not physically allowable and invalidates the hypothesis tested.
Restoration softwares work according to one or the other of the following principles:
1. Restoration by kinematic approach, which applies geometrical transformations to the elements so as to reconstruct the various stages of the deformation. The geometrical transformations applied are calculated or defined by the user, so as to respect the conservation of the volumes (or of the surfaces in 2 dimensions) and contact between the elements,2. Restoration by a mechanical approach, which simulates the subsoil deformation by discretization, then solution of the equations of the mechanics of continuous media. For the purpose of this restoration, the medium is assumed to behave like a quasi-incompressible elastic material; the faults are represented as inner surfaces along which a perfect contact condition applies, without friction. Under such conditions, the calculated displacements and deformations are perfectly reversible, that is the calculation provides the same result whether starting from the final state and going back in time, or starting from the initial state and simulating the deformations in the normal chronological order.
Basin Modeling
Basin modeling softwares allow simulation in one, two or three dimensions all of the sedimentary, tectonic, thermal, hydrodynamic and organic chemistry processes involved in the formation of a petroleum basin. The procedure followed by most basin models comprises three stages:
1. A stage of constructing a grid of the subsoil as it is currently assumed to be, according to a hypothesis on its inner architecture and on the properties that characterize each grid cell: for example their porosity, their sedimentary nature (clay, sand, . . . ) or their organic matter content at the time of their sedimentation. The construction of this model is based on data acquired through seismic surveys or measurements while drilling,2. A stage of reconstructing the prior stages of the basin architecture, based on the hypothesis according to which its deformation only results from a combination of vertical movements by compaction of the sediment or upheaval of its basement. This stage is referred to as backstripping (Steckler, M. S., and A. B. Watts, Subsidence of the Atlantic-type continental margin off New York, Earth Planet. Sci. Lett., 41, 1-13, 1978). It is based on the vertical preservation hypothesis: any vertical segment passing through the basin remains along the same vertical line during the deformation; its length and depth point however evolve over time under the effect of the compaction of the formations and of the subsidence of the basin. It is a form of kinematic restoration that is valid only for the simplest cases of tectonic context;3. A stage of numerical simulation of a selection of physical phenomena taking place during the basin evolution and contributing to the formation of oil traps.
A basin model is a fundamental tool for exploration because it provides all the data required for prediction of the location of reservoirs and of their economic interest.
The basin simulators (softwares) currently available are essentially limited to the hypothesis that the deformation of the basin being studied is correctly represented by backstripping, that is all the deformations can be reasonably represented by vertical movements, without penalizing the reliability of the results. However, many basins actually have a “complex” architecture and tectonic history for which this hypothesis is no longer acceptable; in this context, using a basin model in simple geometry can lead to errors on the prediction of traps. For example, the landslide along some sub-horizontal sedimentary interfaces (referred to as detachment in geology) illustrates the limits of simulations in a simple geometry context. Detachments are very frequent phenomena in the bow areas of the globe. Detachments “shear” the vertical lines of the basin; the subsoil portions located above the detachment are transported horizontally so as to be superposed on others: the vertical preservation condition is no longer valid. This phenomenon can greatly control the current position of the traps. Thus, a vertical migration path can successively fill several reservoirs which are thereafter transported by a horizontal movement to another zone of the basin.